1. Field of the Invention
The invention is in the field of prosthetic dental reconstructions.
2. Description of the Background Art
Dental prostheses are understood here to refer to prosthetic dental inlays, prosthetic dental crowns and bridges, structural elements of all types, such as abutments (spacers), and dental devices of all types, with the invention relating especially to framework structures that may or may not be veneered.
A number of methods involving the production of prosthetic dental inlays or prosthetic dental crowns are known in the art. Generally following the grinding down of the dental defect, an impression is made of the tooth, the area around the tooth, and the jaw; the surface of the cavity may also be recorded on a computer using a stereophotogrammetric scanning device or laser scanning device. The desired external form of the inlay or crown is either reconstructed using a computer-assisted triaxial grinding machine, with the data on the tooth that were gathered and stored prior to the grinding down of the tooth defect, and is then cut directly from a suitable block of material, such as a ceramic block; or is fashioned out of plastic or plaster using models made from the impression that was taken of the untreated tooth.
The conventional technique of using a precious metal or a Ni--Cr alloy to produce an inlay or a crown involves certain aesthetic difficulties, which may be countered by overfiring the metal cap with a ceramic layer. This process, however, is awkward in terms of production mechanics and is susceptible to waste. Additionally, this method makes further monitoring of the condition of the tooth via x-rays impossible, which is considered a disadvantage, particularly in the case of crowns.
The growing demand for aesthetically restorative dental prostheses has advanced the use of other materials and methods, such as high-strength glass, which is poured in liquid form into a refractory mold and is then fired; or the firing of ceramic powder onto a refractory model. Such methods produce inlays and crowns that are aesthetically responsive and characterized by a high degree of x-ray opacity.
Ceramic prostheses that are cut out of ceramic blanks can, however, tend to break off as a result of handling--particularly in their marginal areas, which, in order to yield a seamless transition to the surviving tooth, must be as thin as possible. In addition, grinding traces--which are possible even with precision finishing--can lead to the formation of fissures, and thus to further risk of breakage.
In contrast, a method is specified in U.S. Pat. No. 5,106,303 in which ceramic powder compact is presintered. The form for the inlay or the crown is then profiled from this prepared block of material and is cut in enlarged dimensions to compensate for the shrinkage that will result from the subsequent sintering. Thus, the resulting dental prosthesis will fit into the prepared cavity or onto the prepared tooth stump. The advantage of this method is that the material can be processed in its so-called green state, which makes it easier to work with, since the density and hardness of the material necessary for its use as a dental prosthesis are achieved only after resintering.
Problems can result, however, from the method specified in U.S. Pat. No. 5,106,303 in that during the resintering process, which is performed at a temperature of ca. 1500.degree. C., the very thin marginal areas may become deformed or susceptible to fractures.